ALTDEV ALT OVERSHOT DURING DSCNT PROC ON AUTOPLT.

Narrative

AVOIDING TSTMS ON ARR INTO ATL WE WERE CLRED BY APCH TO TURN R A MAX OF 175 DEG HDG AND DSND TO 10000 FT. WE PENETRATED A CUMULU NIMBUS WITH MODERATE TURB IN THE TURN AND DSCNT PROFILE. DURING THIS SEQUENCE; THE AUTOPLT DISENGAGED FROM CMD (COMMAND) TO CWS (CTL WHEEL STEERING) POS. UPON REALIZATION THAT WE WEREN'T LEVELING OFF AT 10000 FT; DISENGAGED THE AUTOPLT COMPLETELY AND LEVELED OFF AT 9600 FT. I STARTED TO RECOVER TO 10000 AND APCH NOTIFIED US OF OUR RESTRICTION AND THEN CLRED US TO 9000 FT. CAUSE: ASSUME AN AUTO MALFUNCTION. PREVENTION: A C10R (RPT) BEING SUBMITTED TO THE COMPANY PREVENTION. NO LOGBOOK WRITE UP AT THAT TIME BECAUSE OF THE CONFUSION OF THE EVENTS. THE WDB MANUAL STATES THAT ALT PROTECTION IS STILL PROTECTED IN CWS. THE FO AND I; AS A HABIT; VERBALLY CONFIRM ON AN ALT CHANGE THAT THE ANNUNCIATOR PANEL 'ALT ALARM' IS ILLUMINATED. WE BOTH FELT STRONGLY THAT IN THIS INSTANCE; IT WAS ARMED. I SURMISE THAT THE AUTOPLT DROPPED TO THE CWS POS DURING THE ALT CAPTURE MODE AND THIS DISRUPTION CANCELLED THE LEVEL OFF PROTECTION. I CAN FIND NOTHING IN THE PRM ADDRESSING THIS SITUATION. NOTE: AN AUDIO WARNING OF THE AUTOPLT DROPPING FROM CMD TO CWS POS POSSIBLY WOULD ALERT A FLC OF A POTENTIAL HAZARDOUS SITUATION.